The present invention relates to a calibration system for use with digital arrays. Accurate phase and amplitude calibration is critical to achieving low sidelobes and excellent sidelobe cancellation in high performance phased array antennas. Traditional array calibration approaches involve injecting a reference signal via an external source in a far-field or compact range and tuning array phase shifters to optimize a sensitive performance metric. These approaches are tedious and have limited accuracy due to range reflections and limited precision.
Near-field ranges were implemented for high performance measurement and calibration. Near-field ranges have the advantages of high precision and greatly minimized range effects and can provide volumetric pattern data with each near-field scan. For additional detail on planar near-field measurement of digital phased arrays, see article entitled “Planar Near-Field Measurement of Digital Phased Arrays Using Near-Field Scan Plane Reconstruction”, published June 2012 in IEEE Transactions on Antennas and Propagation, Volume 60, Number 6, authors Andrew E. Sayers et al., the entire disclosure of which is incorporated herein by reference. However, an initial difficulty with using a near-field range for calibration was the inability to accurately back-transform from the measured array plane wave spectrum to the element lattice in the aperture plane.
The merged-spectrum technique was used to help address some issues, however, it has an important limitation in that it is only applicable to large arrays for which it can be assumed that all active element patterns are identical. In smaller arrays, sometimes referred to as finite arrays, active element patterns vary from element to element due to variations in the element mutual coupling environments. In these arrays, mutual coupling effects induce amplitude and phase errors which vary from element to element and as a function of scan angle. A method has been needed for highly accurate and efficient finite array amplitude and phase calibration as a function of scan angle. The invention of the present disclosure includes a new system and method which provides these features and is enabled by the implementation and near-field measurement of element-level digital arrays.
Digital arrays are an emerging generation of phased array technology. With either a conventional phased array or Active Electronically Scanned Array (AESA), signals received at each element of the array are phase shifted and combined in an RF combiner to collimate the beam in the direction of interest. The combined signal output of the array interfaces to a receiver. Similarly on transmit, an input signal is split to feed the array elements and a suitable phase shift is provided at each element to collimate the beam in the desired direction. In an element-level digital array, however, receivers are placed at each element of the array and the received signals are converted to streams of digital samples and beam forming and beam steering is performed in the digital processing.
Near-Field Scan Plane Reconstruction is currently used for efficiently measuring element-level digital array antenna patterns. Using this technique, it was shown that any number of far-field volumetric patterns could be obtained from a single planar near-field scan, thus greatly reducing the time required to fully characterize array performance. The limited calibration performance observed in testing programs has created a need for a new calibration technique that is described herein.
According to an illustrative embodiment of the present disclosure, near-field digital array calibration system and method are enabled by digital array technology. The system and method are described as they apply to a digital array operating in the receive mode. However, the system and method are also applicable to a digital array operating in the transmit mode. In general, finite digital array calibration constants can be obtained from the active element patterns of the array. Note that the term active element pattern in the digital array context used here is defined to include all amplitude and phase effects present in the element receive channel as well as amplitude and phase variations resulting from mutual coupling effects. For a given beam steering direction, the array active element patterns across the array yield the amplitude and phase calibration constants for that beam steering direction. Accurate measurement of all of the active element patterns will provide the finite array calibrations over the scan range.
Accurate measurement of individual active element patterns is difficult. Broad active element patterns cannot be measured with sufficient accuracy either in far-field or compact ranges due to chamber effects and other error sources or near-field ranges due to truncation of the near-field scan plane. A key observation is that active element patterns obtained in the near-field range, which are heavily corrupted by the near field scan plane truncation error, nonetheless can be used to provide accurate calibrations. This surprising result is best visualized by considering the near-field scan plane reconstruction technique.
Additional features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of the illustrative embodiment exemplifying the best mode of carrying out the invention as presently perceived.